The present invention relates to a device used which to sense the thickness of strip material in an apparatus for changing rolls of metallized strip material automatically.
In particular, the present invention relates to a sensing device designed to detect the thickness of a strip material consisting at least in part of a diamagnetic material, which is a paper based strip material having at least one face is coated with a metallic film having diamagnetic properties, i.e., having a relative magnetic permeability less than unity. More exactly, such a device is utilized to identify stretches of metallized strip material exhibiting a thickness approximately double the normal thickness. Such a requirement arises where two successive strips are joined together to ensure a continuous infeed of material.
In the field of automatic devices for changing rolls of strip material, considerable importance attaches to a facility allowing automatic detection of the joint between the trailing end of a depleting roll of strip and the leading end of a replacement roll. More specifically, in the majority of applications, it is necessary to discard the portion of joined strip that becomes doubled in thickness as a result of the overlap between the leading and trailing ends. The current method of identifying the joint between two strips is to utilize optical sensors consisting in an emitter element, by which a beam of light is generated and directed at the strip, and a receiver element capable of measuring the intensity of the light penetrating the strip. Given the intensity of the light measured by the optical sensors and the nature of the strip material, it becomes possible to determine when a stretch of double thickness passes between the emitter and receiver elements. Sensors of this type are effective only for certain types of material, however, and in particular, for materials of appreciable transparency. These sensores tend to be substantially ineffective with opaque materials, and altogether ineffective in the case of certain other strip materials exhibiting at least one face coated with a metallic film.
In this latter instance, in fact, the metallic coating completely prevents the passage of a beam of light through the material and the measuring element does no more than verify the presence or absence of the strip.
Accordingly, the object of the present invention is to provide a sensing device capable of operating effectively particularly with a strip material having at least one face metallized.